Hunter Pence is greeted by third-base coach Tim Flannery after the right fielder hit a three-run homer in the first inning.

Photo: Carlos Avila Gonzalez, The Chronicle

Hunter Pence is greeted by third-base coach Tim Flannery after the...

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Colorado Rockies shortstop Josh Rutledge, left, tags out San Francisco Giants' Pablo Sandoval at second base after Sandoval singled during the sixth inning of a baseball game in San Francisco, Monday, April 8, 2013. (AP Photo/Jeff Chiu)

Photo: Jeff Chiu, Associated Press

Colorado Rockies shortstop Josh Rutledge, left, tags out San...

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Angel Pagan scores an insurance run in the eighth.

Photo: Carlos Avila Gonzalez, The Chronicle

Angel Pagan scores an insurance run in the eighth.

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Buster Posey hits an rbi single in the eighth inning scoring Angel Pagan. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Buster Posey hits an rbi single in the eighth inning scoring Angel...

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Madison Bumgarner pitched for the Giants into the sixth inning. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Madison Bumgarner pitched for the Giants into the sixth inning. The...

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Jorge De La Rosa pitches for the Colorado Rockies against the Giants. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Jorge De La Rosa pitches for the Colorado Rockies against the...

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Carlos Gonzalez of the Colorado Rockies reacts after striking out in the eighth inning. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Carlos Gonzalez of the Colorado Rockies reacts after striking out...

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Hunter Pence is greeted at the dugout after hitting his three-run homerun in the first inning. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Hunter Pence is greeted at the dugout after hitting his three-run...

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Sergio Romo, right, and Buster Posey, left, celebrate after the victory. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Sergio Romo, right, and Buster Posey, left, celebrate after the...

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Pablo Sandoval is out at second after he tried to stretch a single into a double in sixth inning. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Pablo Sandoval is out at second after he tried to stretch a single...

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Sergio Romo celebrates after the Giants defeated the Rockies on Monday. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Sergio Romo celebrates after the Giants defeated the Rockies on...

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Angel Pagan calls for time after stealing second in the eighth. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Angel Pagan calls for time after stealing second in the eighth. The...

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Santiago Casilla high fives Buster Posey after retiring the side in the seventh inning. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Santiago Casilla high fives Buster Posey after retiring the side in...

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Sergio Romo, right, and Buster Posey, left, celebrate after the victory. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Sergio Romo, right, and Buster Posey, left, celebrate after the...

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Troy Tulowitzki waits for the throw as Angel Pagan steals second in the eighth inning. The San Francisco Giants played the Colorado Rockies at AT&T Park in San Francisco on Monday, April 8, 2013, and won 4-2.

Photo: Carlos Avila Gonzalez, The Chronicle

Troy Tulowitzki waits for the throw as Angel Pagan steals second in...

The red, white and blue bunting was put away, and the pregame hoopla was limited to some Irish jigs on the first of several ethnic heritage nights at AT&T Park.

Taking a break from celebrating their 2012 title, the Giants got down to baseball for baseball's sake, manager Bruce Bochy saying of the weekend championship pageantry, "Hey, it's over now. ... It's time to go to work."

Hunter Pence wasted no time, smacking a three-run homer in the first inning, which sparked a 4-2 victory over the Rockies on Monday night.

Pence hit seven homers in 59 games with the Giants last year. This season, he has three in seven games, a sign that his value could go well beyond leading postseason pep rallies.

"He's got some of the best power I've ever seen," said Buster Posey, who drove in the Giants' other run for his first RBI of the season. "I get to hit with him in BP, and I'm trying to make sure I hit before him so I don't follow his moon shots."

Madison Bumgarner got the win, though he was far less sharp than in his first start, when he tossed eight shutout innings at Dodger Stadium. He threw 101 pitches that night. On Monday, he needed 104 to get through 5 2/3 innings, issuing his first five walks of the season.

The damage off Bumgarner was Carlos Gonzalez's one-hopper into McCovey Cove in the third inning, a solo homer that came moments after Bumgarner picked off Eric Young, and an RBI single by Chris Nelson, which chased the lefty in the sixth.

"Mental lapses," Bumgarner said. "I didn't feel I had the same trust as the first time. Walks were part of not trusting myself. I didn't have a lot of conviction, I guess. Not the whole night, just on some pitches."

The first-inning rally started on a single by Marco Scutaro, who entered 2-for-23 and admitted he was "jumping to the ball," his timing out of whack. He's dealing with soreness in his back, which "locked up" in spring training and requires daily treatment.

Despite all that, Scutaro has started all seven games.

"He wants to fight his way out of this. He's earned this," said Bochy, who plans to rest Scutaro on Tuesday or Wednesday.